1. Field of the Invention
The present invention relates to a heat exchanger which is typically applied to a condenser or a radiator for a refrigerant cycle in which carbon dioxide is used as refrigerant.
2. Related Art
Recently, refrigerant cycles without using chlorofluorocarbon (hereinafter referred to as flon) as refrigerant are developed to prevent global warming. A super critical refrigerant cycle in which carbon dioxide (CO.sub.2) is used as refrigerant (hereinafter referred to as CO.sub.2 refrigerant cycle) is studied. However, because the CO.sub.2 refrigerant cycle has a high operation internal pressure, heat exchangers used in the CO.sub.2 refrigerant cycle, such as a condenser into which high-pressure refrigerant flows, need to have a high strength. As shown in FIG. 38, JP-A-5-215482 discloses a heat exchanger having plural extruded flat tubes 302. Each of the flat tubes 302 has plural fluid passages 302a having a round-shaped cross-section, so that strength of each flat tube 302 is improved. However, since each fluid passage 302a has the round-shaped cross-section, a wall thickness of the flat tube 302 becomes thicker as compared with a flat tube having fluid passages with a square-shaped cross-section. As a result, weight of each flat tube 302 is increased. On the other hand, when the flat tube has the fluid passages having the square-shaped cross-section, wall thickness and weight of the flat tube are decreased, but strength of the flat tube is also decreased.
On the other hand, JP-A-2-247498 discloses a heat exchanger in which an inner supporting plate is disposed within a header tank having first and second plates, so that strength of the header tank is enhanced. However, in the heat exchanger, the inner supporting plate and the header tank are connected to each other by an acute angle, and stress tends to be intensively applied to a connection portion between the inner supporting plate and the header tank. As a result, the strength of the heat exchanger may be not resistant to high pressures such as 40 MPa of the CO.sub.2 refrigerant cycle.
Further, JP-A-3-260596 discloses a conventional heat exchanger having plural flat tubes 402 through which refrigerant flows, and a pair of substantially cylindrical header tanks 405 connected to both longitudinal ends of the flat tubes 402, as shown in FIG. 39. However, high pressure of the CO.sub.2 refrigerant cycle is approximately ten times larger than that of a refrigerant cycle using flon as refrigerant. Therefore, when the conventional heat exchanger is used in the CO.sub.2 refrigerant cycle, thickness of the header tank 405 may need to be greatly increased so that the header tank 405 has a sufficient pressure resistance. As a result, size and weight of the header tank 405 may be increased.